Some printing systems utilize solid ink that is melted to provide liquid ink for imaging. The solid ink is loaded into the printer and advanced to a melting device which heats the solid ink to a melting temperature. The melted ink is collected and delivered to a printhead and the printhead ejects the melted ink onto media, directly or indirectly, to form an image. Typically, the melting device includes a melting surface that is warmed by a heater to melt the solid ink urged against the melting surface. The melting surface is oriented to enable the melted ink to drain away from the melting surface/solid ink interface. A drip plate receives the melted ink and directs it to a drip point from which the liquid ink drops into a reservoir or other collection vessel for delivery to the printhead. Such a printer is described in U.S. Patent Application US2007/0268348A1 issued to Jones et al. (hereinafter ‘the '348 application’), the disclosure of which is expressly incorporated herein by reference in its entirety.
The temperature of the melt plate must be monitored in order to keep the temperature within a predetermined range. Melt plate temperatures below the predetermined range may indicate a heater that is not generating sufficient heat. Melt plate temperatures in excess of the predetermined range may indicate a heater that is generating too much heat. If the heater malfunctions and the temperature of the melt plate becomes excessive, damage may result to the material being melted, the heater, or other components. Therefore, a thermistor or other temperature sensing apparatus is typically attached to the melt plate. The thermistor senses the temperature of the melt plate and reports the temperature to a controller. When the controller determines that the temperature of the melt plate is above the desired range, the controller takes corrective action (e.g., reduce power to the heater, etc.).
To help protect the temperature sensing apparatus from damage, the primary voltage supplying the heater is isolated from the secondary voltage supplying the temperature sensing arrangement. A dielectric material is commonly used to perform this function of isolating the temperature sensing apparatus from the melt plate. In addition to providing electrical isolation between the melt plate and the temperature sensor, the dielectric material must also have the ability to respond quickly to melt plate temperature changes.
Most heater applications are similar to that described for melting solid ink in that a desired temperature window of operation. Safety, regulatory requirements and thermal control are all pertinent to operation, so details described for an ink melt device are generally applicable to a wide range of heater applications. Prior melt plate and temperature sensor arrangements included a thermistor that was directly soldered to the surface of the melt plate. In these arrangements, the melt plate itself was constructed of a material having good thermal conductivity and good dielectric qualities. One example of such a material is a polyimide film structure, such as a Kapton® laminate. These melt plates typically included a large restricted zone around the temperature sensor intended to isolate the primary voltage delivered to the melt plate from the secondary voltage delivered to the temperature sensor. Heater traces were prohibited in such restricted zones requiring the temperature sensor be physically offset from the most active portion of the heater. The restricted zones resulted in a large portion of unused space on the melt plate and also resulted in some degree of undesirable thermal lag.
In view of the above, it would be desirable to produce a melt plate that does not require a restricted zone. By not requiring a restricted zone, valuable area would be reclaimed on the melt plate, and heater traces could be placed throughout the melt plate. In addition, by not requiring a restricted zone on the melt plate, the thermal lag of the melt plate could be significantly reduced by moving the temperature sense closer to the most active portion of the heater. This reduction in thermal lag would, in turn, increase the power handling capabilities of the melt plate, allowing the melt plate to perform well and survive numerous cycles at increased power levels.